An O-ring is a rubber washer that is round in profile instead of flat. Used in valve stems and push-on fittings to create a watertight seal 

O.C. or on center is even spacing of studs in a wall or other parts in a ceiling or floor by measuring to the center of each component. 

OD orOutside Diameter is the outside dimension of a round pipe or tube

Oil-based paint is a category of paint that includes Alkyds and Enamels and uses oil as a liquid carrier instead of water. Also requires chemical solvents for cleanup

On center refers to the process of placing studs or other framing members centered on a specific repeated spacing in a wall (like 12”,16”, 24”). Sometimes expressed as "O.C."

The on/off switch is an electric switch that supplies current to the automatic shutoff stage of a compressor

The orbital action adjustment is a dial to set more or less circular motion to the cutting action of a reciprocating blade – more orbital motion = faster, rougher, and more aggressive cut

An orbital sander is a tool that uses very small vibrating circular motions to slowly remove material

Organic vapors are liquids that evaporate quickly in a gas and are usually petroleum-based (ex – acetone, paint, gasoline)

OSB stands for Oriented Strand Board which is a Panel made from stranded wood products bound together with glue under high pressure